The lender says the glitches developed following a sweeping computer system enhancement.

"I don't think there is a bigger endeavor that an auto finance company can do than to convert all of their servicing systems into one in one fell swoop," Bob Beatty, executive vice president of customer experience, told Automotive News. "It was as big a project as we could have ever taken on. We knew we would have some issues."

A small number of customers with automatic payment plans -- less than 1 percent of GM Financial's portfolio -- were either under- or overcharged on their payments this month. The average error was $20 above or below the customer's monthly payment, Beatty said.

"We take this very seriously. That's something obviously that a financial institution can't abide," Beatty said, adding: "Customers will be getting refunds of any overage that we withdrew from their accounts, and we'll work with customers on the ones where we withdrew too little."

On each due date this month, GM Financial found several hundred accounts that were affected. By the end of today, the last due date of January, GM Financial will have identified all affected customers, Beatty said.

System upgrade

GM Financial sent a letter to customers explaining intermittent system and phone outages following the system upgrade that began in late December.

The phone system and website were scheduled to be shut down from Dec. 29 to Jan. 5, but the shutdown was extended until Jan. 8.

Because customers couldn't make payments between Dec. 30 and Jan. 8, any payment received from Jan. 8 to Jan. 12 was posted with an effective date of Jan. 1, Beatty said.

GM Financial will correct inaccurate late fees and interest charges and prevent negative credit bureau reports that were driven by the system outage.

"We take this very seriously that no customer is going to be negatively impacted as a result of our technology challenges," Beatty said.

Phone system issues

The lender is still having issues with the phone system, he said. Call volume Tuesday was 300 percent higher than average. "After not being available for a week to your customer base, understandably there were customers that had concerns and wanted to get information," he said. "We have to begin the work of regaining a lot of our customers' confidence."

Beatty said the staff is working around the clock to fix the intermittent phone problems and he is confident they will be resolved "in short order."